A major function of the immune system is to protect the body from
infection and the diseases caused by infectious agents. The immune system also provides protection against
cancer cells, for once they arise,
cancers can essentially behave as "foreign" cells capable of causing pathology. In contrast,
allergy is a manifestation of the immune response to certain environmental cells or molecules that are usually neither a threat for
infection nor
cancer.
Allergic reactions are generally an annoynance, even life-threatening. I will focus on type I
allergy, characterized in part by induction of
IgE antibody responses to
allergens. It should be noted that not all
IgE responses cause allergic symptoms. There is even evidence that
IgE responses to tropical helminthic parasites offer a degree of immunity to
reinfection. I have three objectives: (1) review T cell differentiation leading to the Th1/Th2 paradigm; (2) evaluate the increased prevalence of atopy, including
asthma, as a consequence of a Th2-dominated immune system; (3) relate the high prevalence of
asthma in inner city United States black children to the relatively recent migration of their ancestors from tropical regions of Africa, where genetically biased Th2-dependent
IgE responses may be important in protection against high burdens of parasitic worms.